The present invention relates to methods and apparatus for producing a curable, filled resin composition, and more particularly to methods and apparatus for producing such a composition that is used to make synthetic stone, such as synthetic marble or onyx.
Integral sink basin and countertop structures are formed in a mold from a mixture of a curable synthetic resin and a filler. A pigment is added to the mixture prior to placing the mixture in the mold. The pigment is only partially mixed into the resin and filler so that streaks of pigment appear in the final molded object. If the pigment is properly incorporated into the mixture, the pigment streaks give the molded object the appearance of marble or other real stone product. Such mixtures of filler, resin and pigment when cured are commonly referred to as synthetic stone, and if appropriately pigmented are referred to as synthetic marble or onyx. A common filler employed for a mixture that is to be used to produce synthetic marble is calcium carbonate. A common filler employed to produce synthetic onyx is a combination of aluminum trihydrate and glass frit. A common resin employed to produce synthetic stone products is a polyester resin containing a cross-linking monomer. The molded product resulting from a mixture of polyester resin and a filler is an esthetically pleasing, durable substitute for ordinary countertops into which are inserted ceramic or steel sink basins.
Heretofore, the molding compositions comprising resin and filler have been mixed in a batchwise manner to produce a molding mixture. Ordinarily, commercially available bread dough mixers are employed for the batchwise mixing. Normally, uncatalyzed resin is first placed in the mixing container. Catalyst is then added to and thoroughly mixed with the resin. Thereafter, the filler is placed into the mixing container of the dough mixer. The resin and filler are then mixed until the resin is evenly dispersed throughout the filler. Normally, several minutes are required to evenly disperse the resin throughout the filler when using a dough mixer. When the resin is dispersed, a measured amount of pigment is added to the mixture and is partially distributed or streaked through the mixture, leaving distinct portions of the mixture free of pigment. Thereafter, the mixing container is removed to a location where the molding mixture is poured into a mold and allowed to cure.
Normally the resin is catalyzed so that it will begin to cross-link shortly after it is poured into the mold. Consequently, a new batch of resin cannot be mixed in the container used for a previous batch as the residue of the molding mixture in the container will have cross-linked and thus solidified. Before the container can be used again, it, as well as the mixing tool, must be thoroughly cleaned with solvent. Thus the batchwise processes are not labor efficient because the filler and resin must be manually added to the mixing container, because the time required for the resin to be thoroughly mixed throughout the filler is quite long, and because cleaning time is required after each batch is poured into a mold. Moreover, since a substantial amount of time is required to disperse the resin throughout the filler when prior art mixing methods are used, the resin/filler mixture may begin to cross-link prior to the time it is thoroughly mixed and placed in a mold.
Another problem associated with prior art methods of producing synthetic marble arises because the pigments employed are normally solid materials. In order to obtain the desired marbled effect in the molded product, the pigments necessarily must be mixed with a liquid in order to be properly incorporated into the resin/filler mixture. Normally the pigment is dispersed in a saturated resin, that is, one containing no cross-linking sites, because the saturated resin is compatible with the resin/filler mixture and because the saturated resin will not prematurely cross-link and render the resin/pigment mixture unusable. However, when the saturated resin/pigment mixture is incorporated into the resin/filler mixture, the saturated resin many times is insufficiently dispersed into the resin/filler mixture, leaving regions in the molding mixture where the resin will not cross-link and thus creating undesirable soft spots in the final cured product. If attempts are made to better disperse the saturated resin into the resin/filler mixture, the pigments carried by the saturated resin are often too sparsely distributed throughout the resin/filler mixture, thus again not achieving the desired aesthetic effect in the final product.
Accordingly, a broad object of the present invention is to provide methods and apparatus for continuously producing a curable, filled resin composition, usable for example in making molded synthetic marble articles. Another object of the present invention is to provide methods and apparatus for producing a filled resin composition that eliminates the need to handle the resin and filler on a batchwise basis and that essentially eliminates the lengthy mixing time required by prior art processes to thoroughly disperse the resin throughout the filler. A corollary object of the present invention is to provide methods and apparatus for producing a filled resin composition that will uniformly disperse the resin throughout the filler in a very short time. Another object of the present invention is to eliminate the labor requirements of cleaning the mixing equipment after each object is molded as required by the prior art batchwise mixing. It is still a further object of the present invention to provide methods and apparatus in the use of which the pigment can be mixed with a curable resin and thereby eliminate the occurrence of soft spots in a molded object caused by concentrations of saturated resin. Other objects of the present invention are to provide methods and apparatus for continuously producing a filled resin composition that will increase the overall efficiency of production of objects molded from the composition, and that will decrease the cost per molded object produced.